In a nova, it's fusion of hydrogen. In a type Ia supernova, it's also fusion, but of carbon. But in a core-collapse supernova, the kind massive stars do, the energy source is gravity, as the core collapses. As a minor nitpick, we normally would say energy is conserved, so is not "created" in these events-- it is taken out of the mass of the object.

Pretty much what Ken G said. After a long productive life, the star begins to run out of fuel. As hydrogen runs out, the star must use helium and other gases to quench its thirst. Basically, this doesn't really work out for the star, and collapses under its own gravity because It cant keep burning properly. This collapse can either cause two things, one is a dwarf and the other a supernova. The explosion of a supernova is a build up of all the potential energy stored up as the star burned, which is released as kinetic energy when the fuel runs out. Depending on a number of things, mainly the size of the star, one or the other will happen. If a supernova explodes it can create either both or one of the following: Nebulae and a Black Hole. Generally the burned out core collapses into a black hole, and the surrounding gas turns into nebulae. Nebulae itself in time can form new stars or even planets. Hope this gives you all you need to know.